Intent to Ship: Escape "<" and ">" in attributes when serializing HTML

[Tom Schuster]

Summary: Escape "<" and ">" in attributes when serializing HTML
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1962084
Specification: https://github.com/whatwg/html/pull/6362
Standards Body: HTML WG
Platform coverage: All
Preference: dom.security.html_serialization_escape_lt_gt
Devtools bug: N/A

Other browsers:
- Blink: Rolling out
https://groups.google.com/a/chromium.org/g/blink-dev/c/gE6K8-jDWyE/m/NrI8h2fFAAAJ
- WebKit: Implementing https://github.com/WebKit/WebKit/pull/44842

web-platform-tests: https://github.com/web-platform-tests/wpt/pull/27501
Gecko-only tests were introduced: N/A

Chrome didn't observe any web compat issues in their partial rollout
and I haven't seen any reports for Firefox Nightly either.